Why do people sometimes tap their finger exactly 5 times during contemplation?

I can't be the only one who notices this, but I never hear about it. Sometimes when people are working out a problem that's not too pressing, they will tap their face, temple, or desk in front of them EXACTLY 5 times. Every once in a while you'll see 3, but mostly it's 5. Sometimes you'll see all the fingers on one hand go into a piano playing like gesture. I notice it's only for non serious problems, like what they should have for lunch, or how can they arrange the furniture to impress someone, - those sort of problems. It's a strange little quirk I've always wondered the meaning of. Why is the nervous system designed this way?